A Case Study of Formation Testing in Antelope Shale

Abstract

In new shale development, formation testing is critical to understanding reservoir properties and producibility. However, due to low permeability, a test usually takes much longer and can easily result in cost overruns. There are many ways to conduct formation testing such as drill pipe / tubing conveyed testing and coil-tubing or wireline testing. Formation wireline testing has advantage not only for its flexibility and combinable features but also for cost and time savings as compared to other methods. In a combination way, formation wireline tool can be run either through wireline alone or through the drill pipe for safety reasons. The Antelope Shale in Monterey Formation in San Joaquin Valley is siliceous shale that is thinly laminated, has relatively high porosity, low permeability, small pore throats, and varying degree of fracturing. Siliceous shale hydrocarbon reservoirs are not very common and little is known about their production characteristics. These are much more geologically complex than the conventional shale and tight rock reservoirs and the traditional conventional formation testing methods may not be directly applied to them. Little or no literature review can be found on running formation evaluation wireline tools in the Antelope Shale. In this paper, we discuss a case study of formation testing in Antelope Shale. The test was run with dual packers and downhole fluid analyzer in a vertical appraisal well to evaluate two intervals in the Antelope Shale. The run proved the value of formation testing method in terms of data collected versus cost & time. First, the drill pipe conveyed formation testing was selected among different methods to help characterize the reservoir and to measure fracture closure pressure to evaluate technologies that can lead to the development of tight reservoirs. Second, the microfracturing data collected from the formation testing job was used for designing the completion strategy to understand individual zone production such that we may target a single zone for future development. The additional values of the formation test were to measure formation pressure and collect fluid samples for Pressure, Volume, and Temperature (PVT) analysis to minimize uncertainties in key reservoir parameters. The job proved its pivot values for formation testing. However, the experience from planning and running the tool in this new tight rock reservoir are much more important to achieving the appraisal objectives. The combinable features of the tool enabled continuous onsite monitoring, on-the-fly operational decision making in sample depths in response to formation behavior, and optimizing sample chamber opening time to collect critical oil and water samples, while successfully acquiring the fracture closure pressure. These results may be difficult to achieve with other methods in a single run.